High-Gain Compact SIW-Based Quadruplex Filtering Antenna With Small Frequency Ratio

This letter presents a high-gain compact quadruplex filtering antenna with a small frequency ratio (FR) based on a two-layer substrate integrated waveguide (SIW) structure. The lower-layer feeding section consists of two isosceles right-triangle SIW cavities sharing a common hypotenuse, where independent excitation of the TE₁₀₂ and TE<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink